The present invention relates generally to audio sound systems and more specifically concerns audio sound systems which decode from two-channel stereo into at least four channel sound, commonly referred to as "surround" sound.
Surround systems generally encode four discrete channel signals into a stereo signal which can be decoded through a matrix scheme into the discrete four channel signals. These four decoded signals are then played back through loudspeakers configured around the listener as front, left, right and rear. This principle was adopted originally by Peter Scheiber in U.S. Pat. No. 3,632,886 specifically for audio applications, and the method of encoding four discrete signals into two and then decoding back into four at playback has become commonly known as "quadraphonic" sound. Scheiber's original surround system produces only limited separation between adjacent channels and therefore requires additional dynamic steering to enhance directional information. The basic principle has been applied very successfully in cinematic applications, configured in front-left, front-center, front-right and rear surround, commonly known as Dolby Stereo.TM.. The front-center speaker is designed to be positioned behind the movie screen for the purpose of localizing dialogue specifically from the movie screen. The front-left and front-right channels provide effects, while the rear or surround channel provides both ambient information as well as sound effects. The Dolby Pro Logic.TM. system, a Dolby Stereo.TM. system adapted for home use, uses a tremendous amount of dynamic steering to further enhance channel separation, and is very effective in localizing signals at any of the four channels as an independent signal. The Dolby system, however, provides limited channel separation with composite simultaneous signals.
Although highly effective for audio/video applications, the Dolby Pro Logic.TM. system is not the most desirable for exclusive audio applications. The rear surround channel is limited to 7 KHz, and it does not provide an acceptable amount of low frequency information. The mono center channel, while perfectly suited for dialogue in theater applications, is not desirable for exclusive audio. The center channel has the effect of producing a very mono front image.
It is desirable to provide a multi-channel scheme which can produce four directional channels of information designed specifically for high quality audio applications. It is also desirable that the system have the capability to generate its four directional signals directly from a standard two-channel stereo recording, therefore eliminating any requirement for encoding.
One of the most desirable applications for a system such as this would be automotive sound, configured as left/right front, and left/right rear. Current automotive audio systems send the same left/right information to the rear as is fed to the front. This produces a psycho-acoustic illusion of four channel sound due to the fact that the human ear has a different frequency response to signals directed from the front than it has to signals directed from the rear. For this reason, the current four-speaker stereo system used in automotive applications sounds much more desirable than attempting to adapt a current surround system, such as Dolby's Pro Logic.TM., to automotive applications. Furthermore, there are some major drawbacks to adapting a system such as Dolby's. Since only difference information would be fed to the rear speakers, the rear channel would have a bandwidth of only 7 KHz, and it would be mono in that there would be no directional information perceived to the rear of the listener. As a result, in comparing adapted Dolby Pro Logic.TM. with conventional four-speaker stereo, many listeners would prefer the sound imaging of the conventional four-speaker stereo system.
The majority of the steering schemes devised to enhance directional information have been designed to enhance the normal left, right, center and surround information in a similar fashion to the Dolby Pro Logic.TM. system. For example, using a scheme such as that disclosed by Peter Scheiber, to further enhance directional imaging from a signal previously encoded, David E. Blackmer, in U.S. Pat. No. 4,589,129, provides a discrete rear left, right and center surround channel system. This system is further enhanced for encoding aspects in U.S. Pat. No. 4,680,796 which was also devised specifically for video applications. In U.S. Pat. No. 4,589,129, a very elaborate compression/expansion scheme for encode and decode is disclosed for the purpose of providing noise reduction. However, a major drawback is encountered in this scheme in that the directional steering process is performed broadband and, in the event that predominant steering information is present, objectionable pumping effects are perceived by the listener. This system also has little serious impact in high quality audio applications, due to the fact that the left and right surround information is processed through comb filters. Should a signal be processed by the left or right surround channels, where the fundamental frequency of that signal falls into the notch of one of these comb filters, it would reduce any impact of that signal appearing at the left or right output. Morever, the comb filters will destroy any possibility for side imaging from a system in which a common signal appears at the front and rear of either side, as the rear signal will no longer have the same phase characteristics as the front signal. In addition, if the comb filter is generated with time delays, it would not have the same time domain aspects.
An additional drawback to this system is that it does not lend itself to automotive applications because the surround information is generated strictly by the difference from left and right and there is typically no low frequency energy present in the difference information signal. In automotive sound systems, the majority of the bass is derived from the rear channels because the rear speakers are typically larger and the acoustic cavity in which the speakers are enclosed can typically be much larger and thus provide better bass response.
With the success of Dolby Pro Logic.TM., which has become a standard feature on commercial audio/video receivers, many manufacturers have attempted to provide additional surround schemes that can be specifically applied to audio. In particular, these schemes have added artificial delays and/or ambient information to the rear of the listener. More sophisticated and elaborate systems have been devised and implemented in which the signal is processed through DSP or Digital Signal Processing. Virtually all the attempts made in DSP have also included the addition of artificial reverberation and/or discrete delays to the rear speakers. The addition of information not present in the source signal is not desirable, as the music that is then perceived no longer accurately reflects its original intended sound.
While DSP holds much promise for the future, it is a very expensive system by today's standard and it is desirable to provide a system that could be integrated, incorporating the advantages disclosed, for perhaps one-tenth of the cost of such a system implemented in DSP.
In light of the prior art, and the drawbacks of attempting to adapt any of the prior art systems specifically to automotive applications, it is a primary object of the present invention to provide four-channel sound which greatly enhances the conventional four-speaker stereo system commonly used in auto sound systems. It is also an object of the present invention to achieve a system that requires decode-only for use in high quality audio sound systems which receives an input from a conventional stereo signal, thus allowing for compatibility with all stereo recorded material, and decodes from this two-channel stereo signal an audio sound system incorporating at least four speakers located left/right front and left/right rear. In particular, it is desirable to be able to improve the ambient perceived to the rear of the listener. It is also an object to provide rear directional information without the necessity of adding any artificial information such as delays, reverb, phase correction or harmonics generation that is not already present in the original source material. It is also desirable to provide steering aspects to further enhance left/right directional imaging to the rear of the listener without encountering the objectionable pumping perceived with a single-band system. Furthermore, it is an object to provide emphasis to one side for directional enhancement while providing an increased amount of de-emphasis to the other side. It is also an object to provide discrete left/right imaging to the rear without the necessity of providing comb filters disposed at the audio path, due to the fact that comb filters do not provide results considered to be musically pleasing in high quality audio applications. It is another object of the invention to provide the possibility of localizing simultaneous images to the rear speakers, i.e. a given signal can be perceived as coming from the left while another signal is simultaneously coming from the right. Another object of the present invention is to provide sufficient bass information to the rear speakers of the auto sound system since the majority of the bass delivered in automotive sound is generated from the rear. A further object of the invention is to define a system that can also lend itself to future DSP applications that can further enhance the basic concept of the present invention.